Experiencing volt-drop within household

[bridav]

Hi All,

I am not a qualified electrician - please be gentle!

I had renovation done to my house last October which included a complete refitted kitchen. The new kitchen included a new Neff double oven in which has been working like a charm since last it was installed (including both ovens being on at the same time!).

Suddenly, yesterday the double oven has decided to stop working, checked the distribution board and nothing has tripped. I have tried getting in touch with the electricians that did the installation, but has failed to get back in touch. Therefore, decided to investigate the matter further. I put my multi-meter over the live and neutral terminals on the oven and it read 150V AC, I have checked the outgoing voltage at the MCB which read 240V AC. I thought I may either have:

1. a bad switch
2. a bad MCB
3. a bad connection
4. may be an earth leak

I have tried changing the switch and the MCB, no change to the volt reading [whilst disconnected from the oven]. I have tried tightening the terminals within the distribution board in case of loose connections, no such joy!

Other observations, I noticed that the cable exiting the oven is BLUE and BROWN whereas the cable leaving the distribution board is RED and BLACK. I cannot find the join for this cable as I fear it may be tucked behind the newly plastered walls and ceilings!

Any assistance or information on how to rectify the above volt drop without punching holes about the house would be much appreciated.

Many thanks,
Brian

 

[Octopus]

Finally got a hold of the electrician that did the initial works, the fault was a "damaged cable" apparently caused by the joiner 8 months ago, although I remain a little skeptical about this! However that said luckily the damaged cable was hidden behind the plasterboard within close proximity of the oven isolator and the cable was re-jointed and the oven is now working again. Although I am curious to why the RCB or MCB did not trip.

I was working when the electricians did the works but did instruct the wife to query with them about the certificates, they did a Billy Whizz (from the Beano) in and out before questions were asked and said to the wife "aye I will be having words with the joiner" then left.






Well, continuous sunshine in Scotland is rare, took the advantage while I could before the Scottish winter settles back in (in about 10minutes from now!)

So were you given a MWC to confirm the cable repair and the tests conducted?

 

[bridav]

No, just a brief explanation to the wife that the cable was damaged, now fixed thanks again cheerio.

 

[Lucien Nunes]

I thought I may either have: ...4. may be an earth leak

Although I am curious to why the RCB or MCB did not trip.

Why would you expect a high resistance connection or severed conductor do that? Unless the result was a general meltdown that destroyed the insulation and caused a short, probably revealing itself in the process with escaping smoke?

As an instrumentation engineer - you'll be aware of the input resistance of your DMM, in which case you can calculate the resistance of, and power dissipation in, the point of high resistance. Most likely, it did get hot at one stage, but soon burnt itself O/C. I am skeptical of the explanation given though!

I recall an electrical engineer telling me .... that you could potentially receive a bigger belt from the neutral than the live!

Go on, explain!

 

[Octopus]

No, just a brief explanation to the wife that the cable was damaged, now fixed thanks again cheerio.

Imho you should have.

 

[bridav]

Imho you should have.

I know, I may pester the builder that hired the sparks to see what he has to say!

Go on, explain!

I cannot recall what he said other than something along the lines of volt drop and the resistance of the person touching the neutral...debatable?!

 

[richy3333]

Not sure of Scottish requirements.

Certification as per BS7671 and in certain circumstances a Building Warrant could be required.

no certificate for work done and a balder-dash explanation makes you wonder if they were sparks?

 

[EastAnglian]

Although I am curious to why the RCB or MCB did not trip.

If the dodgy joint was simply providing a high resistance, which would explain the drop in measured voltage under load then (a) the MCB would not trip because the current being drawn would be well within the rated current of the device and (b) the RCD would not trip because its not a fault current to earth.

BS7671 says all work must be documented and there are model forms therin for various types of work. You should have received a certificate when the work was done originally and also when the corrective work was undertaken.

If the repair to the cable is "not accessible for inspection" (i.e buried in the wall) then the method of jointing cannot have screw terminals which can work loose. The most common forms of maintenance free joints are crimps or suitably rated push-fit connectors with sprung-loaded terminals. The joint must be contained in a box providing mechanical protection and it should have strain relief clips on the incoming and outgoing cables.

 

[Risteard]

I would say that it is obviously a loose connection as it would be an achievement to undersize cables enough at a great enough distance to lose 90V in a house!

You need to get an Electrician to sort this out urgently as it is a fire hazard.